Scientists have been forced to rethink how the human genome turns a single cell into a complex living being following the most intensive study of our genetic code ever undertaken.

The research reveals that genes make up only a tiny fraction of the role played by the 3bn letters that constitute the entirety of the human genome.

Large swaths of the genome, previously dismissed as "junk DNA" because it was thought to serve no practical purpose, have been found to be highly active inside the cells in our bodies. Other sequences of genetic code are thought to be "on standby", awaiting a time further down the evolutionary path when they will be beneficial to human beings.

The scientists claim the findings will have a dramatic impact on their ability to pinpoint how genetic defects trigger diseases. Instead of simply looking for mutations in individual genes, it is certain that defects in other parts of the genome will contribute to complex conditions, among them diabetes and coronary heart disease.

The results, published in Nature today, are the culmination of a $42m, five-year project called ENCODE (ENCyclopaedia Of DNA Elements) involving 80 different scientific teams in 11 countries.

The project set out to examine the human genome in unprecedented detail, to work out every different way in which the genetic building blocks, represented by the letters G, T, A and C, work within the body.

The scientists found that beyond genes lay a multitude of other jobs being done by sequences of DNA. Much of the genetic material is transcribed into molecules that relay information from the genome to the biological machinery of our cells.

"If you think of the letters that make up the human genome as the alphabet, then you can think of genes as the verbs. With this project we're identifying all of the other grammatical elements and the syntax of the language we need to read the genetic code completely," said Manolis Dermitzakis, a scientist on the ENCODE project at the Wellcome Trust Sanger Institute in Cambridge.

The findings highlighted how scientists had become so blinded by the importance of genes that the role of other parts of the genome had largely gone unappreciated, he said.

In the pilot study, the researchers focused on 1% of the human genome, or 3bn letters, which were chosen to represent the entire human genetic code. They aim to examine the rest of the genome over the next four years, streamlining the process to complete it for less than $100m.

By understanding how every letter of the human genome functions in the body, scientists believe they will be able to learn how complex diseases are caused by genetic glitches that build up throughout the genome.